This is the 3453rd most frequent Polish word.
zawodniczką
"Zawodniczką" means "female competitor" or "female athlete" in English.
The sentence uses 'zawodniczką' in the instrumental case as part of a predicate indicating her role as a player.
Ona jest najlepszą zawodniczką w drużynie.
She is the best player in the team.
The word 'zawodniczkę' here is in the accusative case, used as the direct object of the verb 'spotkałem'.
Spotkałem zawodniczkę na zawodach sportowych.
I met the player at the sports competition.
The word 'zawodniczce' is in the locative case, used after the preposition 'o' to indicate the topic of discussion.